The present invention relates to titanium-iron complex oxide pigments producing a yellow color for use in coloring plastics, ceramics, coating compositions, etc. and processes for preparing the same.
Complex oxide pigments of the pseudo-brookite type are known as titanium-iron inorganic pigments which produce a yellow color.
The specification of U.S. Pat. No. 4,036,662 discloses pigments of pseudo-brookite and of pseudo-brookite and rutile in mixture which are represented by Fe2TiO5xe2x80xa2xTiO2 wherein x is 0 to 15. According to the disclosure, the color tone is adjustable by varying the mixing ratio of Fe2TiO5 and TiO2. The specification states that fine particles, up to 1 xcexcm in mean size, are obtained by mixing the materials in the form of aqueous suspensions, dehydrating the mixture and thereafter calcining the mixture at a temperature of 600 to 1100xc2x0 C., and that a yellow pigment is available by making the calcining atmosphere reductive with use of carbon black or a like reducing agent. However, the pigment obtained by the above method still remains to be improved in saturation, tinting strength and covering power for use in coloring plastics and coating compositions of which high functions are required.
Fukuji Suzuki prepared Fe2TiO5 of the pseudo-brookite type by mixing titanium oxide of the anatase type and alpha-iron oxide (hematite) and calcining the mixture of 1100 to 1200xc2x0 C. in the atmosphere [xe2x80x9cShikizai (coloring materials),xe2x80x9d 57(12)652-659, 1984]. Although this method starts to produce Fe2TiO5 at a temperature of not lower than 800xc2x0 C., the reaction mixture needs to be heated to 1200xc2x0 C. for the completion of the reaction. The method therefore has the drawbacks of requiring a high calcining cost for preparing the product at such a high temperature and necessitating much time for pulverization since the reaction mixture has been calcined to a great extent. Additionally, the resulting product Fe2TiO5 has the same composition as the product of the foregoing U.S. patent wherein x is 0, and is insofar in saturation, tinting strength and covering power.
JP-A No. 8-73224(1996) discloses a process for preparing complex oxide pigments of the pseudo-brookite type represented by AlxFe2-xTiObxe2x80xa2yTiO2 wherein 0 less than xxe2x89xa61 and 0xe2x89xa6y. According to the disclosure, intensely yellow finely particulate complex oxide pigments having high saturation are prepared by dispersing or dissolving oxides, hydroxides or water-soluble salts of iron and aluminum in a hydrated titanium oxide slurry, causing the resulting slurry to produce a coprecipitate at a suitable pH, and washing the coprecipitate with water, followed by filtration, drying and calcining at a temperature of about 800 to about 1100xc2x0 C. This process nevertheless involves limitations to starting materials, requires complex equipment and steps and is high in production cost.
Thus, titanium-iron complex oxide pigments of pseudo-brookite are produced by mixing titanium oxide or metatitanic acid with an iron oxide, iron hydroxide or water-soluble iron salt in a predetermined ratio in a dry or wet state, or causing an aqueous solution of water-soluble salts of titanium and iron to undergo coprecipitation and washing the precipitate, followed by drying to obtain a material mixture, and calcining the mixture in air or a reducing atmosphere at a temperature of 800 to 1200xc2x0 C. In the calcining step, Ti, Fe or O from particles of one of the materials diffuse into those of the other, thereby giving rise of a reaction to form and grow pseudo-brookite crystals. To industrially manufacture a stabilized product, especially iron-containing complex oxides, by such a solid-phase reaction, the stability of the materials to be used, the stability of the materials as mixed together and the stability of calcining operation are of extreme importance.
The materials, especially iron compounds, such as iron oxides or iron hydroxides, are easily oxidized or reduced by a slight change in temperature and atmosphere. Accordingly, it is difficult to obtain materials which always exhibit constant reactivity during calcining, hence limitations of the materials usable. Further when materials are mixed together which are different in specific gravity, particle size or bulkiness, segregation is liable to occur in the mixture being prepared by mixing or as prepared by mixing owing to the difference in specific gravity or particle size, presenting difficulty in affording a material mixture of uniform composition.
Further since mass transfer between the particulate materials is a rete controlling factor of the reaction, low temperatures lead to a low reaction rate to result in low productivity, quantity manufacture of the desired product on an industrial scale requires a high temperature for calcining.
Because the iron oxide or iron hydroxide in the material mixture is easily oxidized or reduced by a slight change in the temperature or atmosphere, a product of uniform color is not available unless the calcining temperature and atmosphere are controlled closely.
It is known well that when iron-containing complex oxide pigments are to be manufactured with good stability and with uniform color produced, it is necessary to use materials of high quality as controlled strictly, to hold the materials as uniformly mixed at all times and to calcine the mixture with the temperature, time and atmosphere controlled strictly. These requirements make the equipment and process complex and result in an increased manufacturing cost.
In view of the foregoing problems, an object of the present invention is to provide a yellow pigment which contains a titanium-iron complex oxide of the pseudo-brookite type and produces a stabilized color more yellowish than conventionally and which is universally usable for coloring plastics, ceramics and coating compositions.
The present applicant previously proposed a process for producing a yellow pigment of the rutile type by mixing titanium oxide, oxide of one of Co, Cr and Ni and oxide of one of Sb, Nb and W, grinding the resulting composition by a dry method using a mill to prepare composite particles by utilizing a mechanochemical reaction, and calcining the particles [see JP-A No. 10-219134 (1988)]. The process is adapted for the production of complex oxide pigments of the rutile type such as Ti-Sb-Cr, Ti-Sb-Ni, Ti-Nb-Co or Ti-W-Ni.
The present inventors have subsequently conducted research for the production of yellow pigments and found that a pseudo-brookite complex oxide pigment of outstanding characteristics can be obtained by applying a mechanochemical reaction to titanium-iron material particles to prepare composite particles, followed by calcining.
Stated more specifically, the present invention provides a first complex oxide pigment, i.e., a titanium-iron complex oxide pigment which is characterized in that the pigment contains a pseudo-brookite complex oxide represented by:
the composition formula (M1-xxe2x80xa2Fex)Oxe2x80xa2TiO2 or
the composition formula (Fe1-yxe2x80xa2Aly)2O3xe2x80xa2TiO2 
wherein M is one of the bivalent metals Mg, Sr and Z, the ratios of Fe, Al and M to Ti are in the respective ranges of 0.3xe2x89xa6Fe/Tixe2x89xa64.5, 0xe2x89xa6Al/Tixe2x89xa66.5, and 0xe2x89xa6M/Tixe2x89xa62.6, and x and y are in the respective ranges of 0xe2x89xa6x less than 1 and 0xe2x89xa6y less than 1.
The present invention further provides a process for producing the first complex oxide pigment. This process is characterized by mixing particulate starting materials for Ti, Fe, Al and M in specified proportions, grinding the resulting particulate composition in a dry state to give the composition energy sufficient to cause a mechanochemical reaction, join the particles to one another and prepare composite particles wherein the elements Ti, Fe, Al and M are present, and calcining the composite particles at 700 to 1200xc2x0 C.
The present invention provides a second complex oxide pigment, i.e., a titanium-iron complex oxide pigment which is characterized in that the pigment contains a pseudo-brookite complex oxide, and has added thereto at least one element selected from the group consisting of Li, B, Si and Ca, the pseudo-brookite complex oxide being represented by:
the composition formula (M1-xxe2x80xa2Fex)Oxe2x80xa22TiO2 or
the composition formula (Fe1-yxe2x80xa2Aly)2O3xe2x80xa2TiO2 
wherein M is at least one metal selected from the group consisting of the bivalent metals Mg, Sr and Zn, the ratios of Fe, Al and M to Ti are in the respective ranges of 0.3xe2x89xa6Fe/Tixe2x89xa64.5, 0xe2x89xa6Al/Tixe2x89xa66.5 and 0xe2x89xa6M/Tixe2x89xa62.6, and x and y are in the respective ranges of 0xe2x89xa6x less than 1 and 0xe2x89xa6y less than 1. The second complex oxide pigment remains free of discoloration even when mixed with resins with heating and has high heat resistance.
The present invention further provides a process for producing the second complex oxide pigment. This process is characterized by mixing particulate starting materials for Ti, Fe, Al and M, and at least one element selected from the group consisting of Li, B, Si and Ca in specified proportions, grinding the resulting particulate composition in a dry state to give the composition energy sufficient to cause a mechanochemical reaction, join the particles to one another and prepare composite particles wherein the elements Ti, Fe, Al and M, and at least one element selected from the group consisting of Li, B, Si and Ca are present, and calcining the composite particles at 700 to 1200xc2x0 C.
Instead of the conventional method of mixing the particulate starting materials (source substances for the elements providing the first or second complex oxide pigment) in predetermined proportions and thereafter treating the mixture wet or dry using a mixer, the process of the present invention grinds the particulate starting materials in a dry state with use of a mill having a high grinding efficiency to pulverize and mix the particulate materials. The dry grinding treatment is thereafter continued further to give the particulate materials great mechanical energy, such as that of grinding, friction, compression, tension, bending and collision, not less than is required for grinding, whereby the material particles which are pulverized and uniformly mixed together are joined to one another to become greater in size, thus producing a phenomenon termed xe2x80x9cinverse grinding.xe2x80x9d In this way, composite secondary particles are formed wherein the elements incorporated into the starting composition are present uniformly in a definite ratio. Along with this phenomenon, the starting materials diminish in crystallinity and become partly amorphous. This is what is termed a mechanochemical reaction.
When the dry grinding treatment of the particulate starting materials resorting to the mechanochemical reaction is conducted for a longer period of time, the increase of particle sizes due to inverse grinding and the decrease of particle sizes due to grinding take place at the same time, so that the treatment has the feature that variations in particle size reach equilibrium. Accordingly, even if the starting materials differ in specific gravity, particle size or bulkiness, composite secondary particles can be obtained always as stabilized in particle size and with a uniform composition.
The composite secondary particles obtained by the mechanochemical reaction are not in the form of a mere mixture but contain all the elements incorporated into the starting composition, uniformly and compactly in a definition ratio, are therefore very high in reactivity and can be calcined at a lower temperature within a shorter period of time, affording a pigment which is exceedingly higher in saturation and more excellent in tinting strength than those obtained by calcining a mixture obtained by the conventional method of wet or dry mixing.
The process of the present invention merely grinds the starting materials in a dry state using a mill instead of mixing the starting materials in the conventional manner, whereby the foregoing object can be accomplished, consequently necessitating no complex production step such as coprecipitation. Furthermore, the composition can be calcined at a lower temperature within a shorter period of time, and the calcined product can be pulverized easily. The process is therefore advantageous also in cost. Although the conventional material mixing method encounters difficulty in preparing a mixture of uniform composition from materials which are different in specific gravity or particle size, starting materials which are different in particle size or specific gravity can always be made into composite secondary particles of uniform composition by the dry grinding treatment of the process of the invention. This eliminates limitations of the starting materials.
The mechanochemical reaction per se is already known as disclosed in Kiichro Kubo, xe2x80x9cMechanochemistry of Inorganic Materials,xe2x80x9d (published by Sogogijutsu Shuppan, 1987), and it is disclosed that the reaction is applicable to the surface modification of particles and to the precipitate of high-temperature superconductive substances, whereas the application of the reaction to the preparation of pseudo-brookite complex oxide pigments still remains to be disclosed.
The starting materials for use in preparing the first complex oxide pigment, i.e. the source substances for the elements providing the pigment, may be, for example, oxides, hydroxides or carbonates of Ti, Fe, Al and M (which is at least one metal selected from the group consisting of the bivalent metals Mg, Sr and Zn), or compounds of these elements which become oxides when heated. Generally useful Ti sources are titanium oxide of the anatase type, titanium oxide of the rutile type, metatitanic acid (hydrated titanium oxide), etc. Examples of preferred Fe sources are iron oxides, yellow iron hydroxide, iron chlorides, iron nitrates, etc. Examples of useful Al sources are aluminum oxide, aluminum hydroxide, aluminum chloride, aluminum nitrate, aluminum acetate, etc. Examples of preferred Mg sources, Sr sources and Zn sources are oxides, hydroxides, carbonates, chlorides, nitrates, etc. of these metals.
Among the starting materials for use in preparing the second complex oxide pigment, i.e. among the source substances for the elements providing the pigment, the source substances for Ti, Fe, Al and M are the same as those used for the preparation of the first complex oxide pigment. Useful source substances for at least one element selected from the group consisting of Li, B, Si and Ca may be the oxide, hydroxide, carbonate, chloride, nitrate, etc. of the element.
Since the composite secondary particles obtained by the dry grinding treatment of the starting materials are not susceptible to the influence of the specific gravity, the particle size or the bulkiness of the starting materials, any of these materials can be widely varying in particle size or bulkiness, and satisfactory to use are inexpensive materials which are generally used widely. Examples of starting materials which are preferable from the viewpoint of cost and quantities to be supplied are titanium oxide of the anatase type or rutile type or metatitanic acid, ferrous oxide, ferric oxide, yellow iron hydroxide, aluminum oxide, magnesium carbonate, strontium carbonate, zinc oxide, lithium carbonate, boric acid, silicon oxide, calcium carbonate, etc. which are not surface-treated.
In producing the first and second complex oxide pigments, the source substances for Ti, Fe, Al and M are used in a ratio which is so determined that the ratios of Fe, Al and M (at least one metal selected from the group consisting of the bivalent metals Mg, Sr and Zn) to Ti in the following composition formula:
(M1-xxe2x80xa2Fex)Oxe2x80xa22TiO2 wherein Cxe2x89xa6x less than 1, or 
(Fe1-yxe2x80xa2Aly)2O3xe2x80xa2TiO2 wherein 0xe2x89xa6y less than 1 
will be in the respective ranges of 0.3xe2x89xa6Fe/Tixe2x89xa64.5, 0xe2x89xa6Al/Tixe2x89xa66.5 and 0xe2x89xa6M/Tixe2x89xa62.6.
Ti and Fe are essential components. If Fe/Ti less than 0.3, the content of the chromophore component Fe is too small to produce the color. If Fe/Ti greater than 4.5, an excess of Fe is present, with the result that the product becomes turbid yellow with reddish brown incorporated into the color owing to the formation of Fe2O2 in addition to pseudo-brookite crystals. More preferably, the range is 0.4xe2x89xa6Fe/Tixe2x89xa63.5.
The addition of Al is effective for adjusting the color tone, whereas if Al/Ti greater than 6.5, the product appears reddish and is impaired in tinting strength and covering power. The ratio is more preferably in the range of 0.1xe2x89xa6Al/Tixe2x89xa65.
Although Mg, Sr and Zn are not chromophore elements, addition of these elements makes it possible to adjust the density of color to be produced and the color tone.
If M/Ti greater than 2.6, increased amounts of M and Ti are present in the composition, consequently reducing the relative amount of the chromophore component Fe and rendering the pigment unable to produce a satisfactory color. More preferably, the range is 0xe2x89xa6M/Tixe2x89xa62.
In producing the second complex oxide pigment, the proportion of the source substance to be used for at least one element selected from the group consisting of Li, B, Si and Ca is determined preferably in the following manner.
The proportion of the source substance to be used for Li is so determined that Li is in the range of 0.07xe2x89xa6Li2O/Nxe2x89xa60.75 (ratio by weight) relative to the pseudo-brookite complex oxide (N), and that Li has the relationship of 0.015xe2x89xa6Li/Fexe2x89xa60.074 with Fe. If 0.07 less than Li2O/N (ratio by weight), the effect to achieve improved heat resistance is small, whereas when Li2O/N greater than 0.75 (ratio by weight), impaired heat resistance will conversely result. The ratios are more preferably in the respective ranges of 0.07xe2x89xa6Li2O/Nxe2x89xa60.17 (ratio by weight) and 0.015xe2x89xa6Li/Fexe2x89xa60.03.
The proportion of the source substance to be used for B is so determined that B is in the range of 0.2xe2x89xa6B2O3/Nxe2x89xa61.20 (ratio by weight) relative to the pseudo-brookite complex oxide (N), and that B has the relationship of 0.015xe2x89xa6B/Fexe2x89xa60.05 with Fe. If 0.2 less than B2O3/N (ratio by weight), the effect to achieve improved heat resistance is small, whereas when B2O3/N greater than 1.20 (ratio by weight), the particles are sintered to a greater extent during calcining and become difficult to pulverize, while the product produces a brown color of dark tone. The ratios are more preferably in the respective ranges of 0.2xe2x89xa6B2O3/Nxe2x89xa60.43 (ratio by weight) and 0.015xe2x89xa6B/Fexe2x89xa60.03.
The proportion of the source substance to be used for Si is so determined that Si is in the range of 0.59xe2x89xa6SiO2/Nxe2x89xa64.90 (ratio by weight) relative to the pseudo-brookite complex oxide (N), and that Si has the relationship of 0.024xe2x89xa6Si/Fexe2x89xa60.125 with Fe. If 0.59 less than SiO2/N (ratio by weight), the effect to achieve improved heat resistance is small, whereas when SiO2/N greater than 4.9 (ratio by weight), the particles are sintered to a greater extent during calcining and become difficult to pulverize, while the product produces a brown color of dark tone. The ratios are more preferably in the respective ranges of 0.59xe2x89xa6SiO2/Nxe2x89xa61.8 (ratio by weight) and 0.024xe2x89xa6Si/Fexe2x89xa60.03.
The proportion of the source substance to be used for Ca is so determined that Ca is in the range of 0.55xe2x89xa6CaO/Nxe2x89xa64.50 (ratio by weight) relative to the pseudo-brookite complex oxide (N), and that Ca has the relationship of 0.026xe2x89xa6Ca/Fexe2x89xa60.13 with Fe. If 0.55 greater than CaO/N (ratio by weight), the effect to achieve improved heat resistance is small, whereas when CaO/N greater than 4.5 (ratio by weight), the particles are sintered to a greater extent during calcining and become difficult to pulverize, while the product produces a brown color of dark tone. The ratios are more preferably in the respective ranges of 0.55xe2x89xa6CaO/Nxe2x89xa61.71 (ratio by weight) and 0.026xe2x89xa6Ca/Fexe2x89xa60.078.
Next, a detailed description will be given of the processes or producing titanium-iron complex oxide pigments of the present invention.
The composition of the source substances for the elements of the first or second complex oxide pigment is ground in a dry state. Examples of mills for use in the dry grinding treatment are those having a high grinding efficiency, such as rotary ball mills, tube mills, vibrating mills, planetary mills, medium-agitating mills, shear grinding mills, and high-speed rotary impact mills. These mills may be of the batchwise type or the continuous type. From the viewpoint of enlarging the scale of industrial operation, ease of control and treatment efficiency, vibrating mills and medium-agitating mills are desirable. Examples of grinding media for dry mills for use with such a medium are balls, cylinders, rods, etc. Examples of materials for such media are alumina, zirconia and like ceramics, steel, tool steel and like metals. Balls are used as the grinding medium for vibrating mills, planetary mills and medium-agitating mills. The size of balls exerts an influence on the size of composite secondary particles resulting from the dry grinding treatment and is generally 1 to 30 mm in diameter. The composition is subjected to the dry grinding treatment for a period of time which varies with the mill to be used, with the charge of starting materials and with the quantity of grinding medium. It is desirable to continue the dry grinding treatment until particle size of the charged starting materials does not decrease and the increase in particle size due to inverse grinding and the decrease in particle size due to grinding are brought into equilibrium to result in no variations in particle size.
To prevent the adhesion of particles of the starting materials to the grinding medium during the dry grinding treatment and to effectively give rise to the mechanochemical reaction, a liquid auxiliary agent can be added under the conditions of dry grinding treatment. Examples of auxiliary agents usable for the dry grinding treatment are ethanol, propanol and like alcohols; ethylene glycol, propylene glycol, glycerin and like polyhydric alcohols; diethanolamine, triethanolamine and like alcohol amines; stearic acid; waxes of low melting point; etc. These auxiliary agents are used usually in an amount of 0.05 to 5 wt. % based on the charge of starting materials although the amount varies with the type of auxiliary agent, the particle size of the particulate starting materials and the surface area of the grinding medium. If the amount of the auxiliary agent is too small, particles of starting materials will adhere to the inner wall of the mill or to the grinding medium, hampering the grinding and mixing operation and failing to produce composite secondary particles. If an excessive amount of auxiliary agent is used, the treatment fails to form composite secondary particles although the starting materials are ground and mixed together.
Next, the composite secondary particles are calcined in a usual calcining furnace in the atmosphere at a temperature of 700 to 1200xc2x0 C. for 0.5 to 10 hours. If the calcining temperature is below 700xc2x0 C., a lower reaction rate will result to necessitate too long a calcining time. Further if the calcining temperature is in excess of 1200xc2x0 C., the product becomes sintered to excess, presenting difficulty in adjusting the particle size of the calcined product. Preferably, the calcining operation is conducted at a temperature of 800 to 1100xc2x0 C. for 1 to 6 hours. The calcining atmosphere is not limited particularly and may be the atmosphere.
The process for producing the first complex oxide pigment by subjecting the particulate starting materials for the pigment of the pseudo-brookite to the dry grinding treatment makes the particulate starting materials amorphous to diminish the activating energy required to give rise to a solid-phase reaction during calcining, joins the particles to one another firmly, remarkably increases the number of particle-to-particle contact points which provide sites where the reaction is initiated, and further permits all the elements used to be uniformly and compactly present within the particles in a definite ratio. Accordingly, the process assures very high reactivity and a remarkably increased solid-phase reaction rate. Being subjected to the dry grinding treatment, the particulate starting materials can be calcined at a lower temperature for a shorter period of time than conventionally, affording a pigment with good stability which contains a highly yellowish pseudo-brookite complex oxide having high saturation and outstanding tinting strength.
Furthermore, the second complex oxide pigment having Li, B, Si and/or Ca incorporated therein remains free of discoloration even if mixed with a resin with heating and has high heat resistance.
Thus, the present invention makes it possible to produce yellow pigments which contain a titanium-iron complex oxide of the pseudo-brookite type and which are high in saturation, excellent in tinting strength, intensely yellowish and usable for coloring plastics, ceramics and coating compositions.